1. Technical Field
The present invention relates to mobile robots and, more particularly, to a robot having a spherical exo-skeleton and an internal propulsion mechanism.
2. Discussion
Mobile robots have been studied for decades. Researchers have investigated robot applications ranging from material transportation in factory environments to space exploration. A particularly extensive area of mobile robot application is found in the automobile industry where robots transport components from manufacturing work stations to the assembly lines. These automated guided vehicles (AGVs) follow a track on the ground and have the ability to avoid collisions with obstacles in their path. Autonomous mobile robots designed for planetary exploration and sample collection during space missions have also received significant attention in recent years. This attention has resulted in advancement of mobile robot technology and a corresponding increase in the effectiveness of mobile robots in a wide range of applications.
Mobile robot technology has primarily focused on robot designs having a body with wheels for mobility. This has led to advancements in motion planning and control of the rolling wheel. Notwithstanding these developments, wheeled mobile robots have significant deficiencies that have not been adequately overcome. For example, wheeled robots often times have difficulty traversing rough terrain. While this problem may be reduced by increasing the size of the wheels of the robot, increases in wheel size cause various undesirable consequences including an increase in the overall size and weight of the robot. Unfortunately, increases in wheel sizes do not necessarily result in corresponding increases in operational features such as payload capacity. Also, wheeled robots are adversely effected by harsh operating environments such as heat, chemicals, and the like.
The present invention relates to a spherical mobile robot that moves by rolling over terrain The control of a rolling sphere involves reconfiguration of its position and orientation coordinates. Control strategies developed for wheeled mobile robots do not directly relate to precise control of the reconfiguration of the rolling sphere.
Wheeled mobile robots belong to a class of systems known as nonholonomic systems. Feedback control strategies developed for nonholonomic systems are typically smooth and time varying, piecewise non-smooth and time-invariant, or a hybrid combination of the two. Such strategies, however, are applicable to nonholonomic systems that can be converted into a special form, known as chained form. It has not been possible to convert the kinematic model of the sphere into chained form. Therefore, the above strategies do not lend themselves directly to the reconfiguration problem of the rolling sphere.
Typically, among control strategies for nonholonomic systems, time-varying controllers suffer from slow rates of convergence. Faster convergence rates can be achieved through the design of piecewise non-smooth time-invariant controllers. However, piecewise non-smooth time-invariant controllers may involve multiple switchings and may lead to undesirable chattering. Hybrid controllers are based on switchings at discrete-time instants between various low level smooth controllers. Such controllers tend to combine the advantages of both the time-varying and time-invariant controllers.
While non-smooth control is generally sufficient to provide operational capability to a spherical robot of the type described herein, a non-smooth technique has undesirable characteristics including intermittent motion and chattering. A non-smooth controller is particularly undesirable in the present invention in view of the spherical robot's internal drive mechanism. More particularly, the internal drive mechanism may be limited in its ability to generate the large accelerations necessary for non-smooth control of the system.